$("#upload").live("click", function() {
	var file_data = $("#avatar").prop("files")[0];   // Getting the properties of file from file field
	var form_data = new FormData();                  // Creating object of FormData class
	form_data.append("file", file_data)              // Appending parameter named file with properties of file_field to form_data
	form_data.append("user_id", 123)                 // Adding extra parameters to form_data
	$.ajax({
                url: "/upload_avatar",
                dataType: 'script',
                cache: false,
                contentType: false,
                processData: false,
                data: file_data,                         // Setting the data attribute of ajax with file_data
                type: 'post'
       })
})